Which linkage.
What linkage?
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there is a set screw on the linkage on the carb that will adjust it.
Quite simply it means that the connection between the gear lever & the gearbox is a cable. Other means of connection are. Direct from the gear lever into the gearbox Rod linkage between the gear lever & the gearbox.
Thats fairly easy to do. First block the wheels so the vehicle will not roll. Put the transmission into neutral. Go under the car and on the drivers side of the transmission pan you will see the linkage. Remove the pin holding the linkage to the transmission. The linkage should slide in and out of the transmission shift bracket easily. If not loosen the nut on the linkage and adjust it until the linkage slides in and out without binding.
On the driver's side of the F 150 carb, there should be the linkage for the throttle. On the linkage there should be a set screw that you can turn to adjust the engine idle.
Between your shifter and the gearbox, there is a linkage. A connector is broken.
Could be clutch adjustment or shift linkage adjustment
The 1993 Mercury manual transmission linkage will have adjustment nuts on each end of the linkage. Turning the adjustment nuts will move the linkage back in forth.
you do not have a neutral safety switch it is a valve in the gear box adjust the linkage put in first gear now to the engine bay look at the throttle now adjust the linkage see if gearstick is in the right place the engine must only start in park or neutral if not you must adjust the linkage.
usually a linkage problem - adjust linkage that connects transmission to gear shift